The present invention relates to the method for making end-capped polycarbonates utilizing bisphenol monochloroformate polycarbonate oligomer in combination with a phenol chainstopper.
Prior to the present invention, as shown by Silva et al, U.S. Pat. No. 4,743,676, polycarbonates were prepared by reacting a bischloroformate composition with a monohydroxy aromatic compound in a mixture comprising water, base and a suitable organic liquid, and then converting the resulting partially capped bischloroformate composition to linear polycarbonate by contact with an interfacial polycarbonate formation catalyst in an alkaline medium. Although the method of Silva et al resulted in linear polycarbonates of controlled molecular weight, about half of the bisphenol bischloroformate composition end groups had to be hydrolyzed prior to condensation and formation of polymer. In addition to being hydrolysis limited, the bisphenol bischloroformates, as a result of such hydrolysis requirement, can form by-products, such as carbon dioxide and carbonate salts which require extra water to prevent salt precipitation. In addition, amine/chloroformate complexes, such as, acylammonium salts are formed in excess quantities. These acylammonium salts are surface active and can cause excess emulsification during polymerization. Such emulsions interfere with the efficiency of polycarbonate formation.
In U.S. Pat. Nos. 3,193,528 Miller et al, and 3,354,051 Schmitt, methods are described for preparing monochloroformates of bisphenols or mixtures of bisphenols utilizing phosgene, a bisphenol and dilute aqueous caustic. In copending application RD-19,525, an interfacial method is described for making bisphenol monochloroformate polycarbonate oligomers and a product obtained therefrom. There is shown in RD-19,525, a method of making bisphenol monochloroformate polycarbonate oligomers comprising a mixture of substantially equal molar amounts of bisphenol monochloroformate oligomers having a chloroformate end group and a hydroxyl end group in combination with a substantially equivalent amount of a mixture of bishydroxy terminated bisphenol polycarbonate oligomers and bisphenol bischloroformate oligomers. The aforementioned bisphenol monochloroformate polycarbonate oligomers can be made by phosgenating a mixture of bisphenol under interfacial conditions where the mixtures contain sufficient alkali metal hydroxide to provide a pH of up to about 11.2. As the phosgenation continues, the pH of the mixture continues to drop until a pH set point, for example 8, is reached. Additional make-up aqueous alkali metal hydroxide can be introduced at various rates, until a signal is shown indicating bisphenol monochloroformate oligomer formation whereupon termination of base and phosgene introduction can be effected. If a particular control system is used, for example "Control System A", aqueous alkali metal hydroxide can be introduced into the phosgenation mixture at a rate which substantially maintains the pH of the mixture at the pH set point during phosgenation until a sudden rise in base demand occurs. Alternatively, a "Control System B" can be used which restricts the aqueous alkali metal hydroxide introduction to a rate sufficient to provide a ratio of the rate of moles of aqueous alkali metal hydroxide introduction to the rate of moles of phosgene introduction having a value of up to about 2.5. With Control System B, the pH of the phosgenation mixture is found to cycle around the pH set point until it is found to substantially stabilize and thereafter suddenly falls to at least 1 pH unit below its previous stabilized value. These signals identifying either a change in pH or base flow into the phosgenation mixture establish the threshold point of bisphenol monochloroformate polycarbonate oligomer formation and the point at which phosgenation and base introduction can be terminated.
It would be desirable therefore to provide a method for making end-capped polycarbonates having a predetermined molecular weight without excess emulsification occurring during polymerization. It also would be desirable to use a particular ratio of monofunctional phenol chainstopper and the aforedescribed bisphenol monochloroformate polycarbonate oligomer, to not only reduce the level of emulsion formation but also to reduce the phosgene requirement, by-product diphenyl carbonate formation, and to increase the accuracy of the pH measurement during polymerization.